Metallography: Principles and Practices - ASM International

Metallography: Principles and Practice (#06785G)
Author(s): George F. Vander Voort
26 METALLOGRAPHY
Copyright © 1984 ASM International ®
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Figure 1-17 Dendrites observed on a broken sec-
9X tion of cast iron.
the proper crystallographic orientation will grow an appreciable distance into the
liquid. In body-centered cubic metals, such as iron, the direction of dendrite
growth is always the cube axis, i.e., . The primary dendrite spacing
depends upon the initial freezing conditions in the chill zone and thus is controlled
by the nucleation rate. However, the secondary arm spacing is not a function of
the nucleation rate at the chill surface but is controlled by the growth rate away
from the surface. Thus, the critical factor for the secondary arm spacing is the rate
of heat removal from the casting. The degree of heat removal changes constantly
during most casting processes, and therefore measurement of the secondary arm
spacing is of considerable value in the study of solidification. Although some
qualitative information regarding dendritic spacing was known, Alexander and
Rhines performed the first quantitative study of the solidification process [15]. In
this report Alexander and Rhines discussed the problem of making measurements
of dendrite spacing. In order to eliminate the need for corrections for orientation
effect, they first made spacing measurements only in grains where the major
dendrite axis was nearly in the plane of polish. Since this is a matter of judgment,
some error can be introduced. They observed that not all of the secondary arms
were well developed, and chose to measure only spacings between fully developed
secondary arms. Since this is also a matter of judgment, error can result and the
degree of repeatability of measurements suffers. They also observed that two
characteristic spacings were present in the center of some of their ingots; they
chose to record the larger spacing rather than the smaller or an average of the two.
In some metals, such as aluminum castings, it is difficult to measure dendrite
arm spacings. In these metals, one can measure the dendrite cell size, which is the
width of the individual cells, or the dendrite cell interval, which is the center-tocenter
distance between adjacent cells. In alloys with a small amount of interden-